2b: Measurements of enthalpy change [planning] Flashcards
What are we asked to investigate in this practical
We have to investigate the enthalpy changes accompanying the solution of some ionic solids using a simple calorimeter.
What is the definition of the standard enthalpy of solution
is the enthalpy change when 1 mol of a substance is completely dissolved in an infinite amount of solvent/water, under standard conditions.
What does infinite dilution mean
It means that the solid dissolves in a sufficiently large amount of water so that the dissolved ions are well separated and do not interact with one another.
What is the equation for enthalpy of solution
lattice enthalpy of disassociation plus lattice enthalpy of hydration
or
minus lattice enthalpy of formation plus enthalpy of hydration
What happens when an ionic solid dissolves in water
The ions become mobile, they possess an independent existence in dilute solution. Unlike the relatively fixed positions which they occupy in a crystal lattice.
What is the generic method
1: Measure out your calculated mass of solid
2: Measure out 50 cm³ of water
3: Measure the initial temperature
4: add the solid to the water
5: ensure the reaction goes to completion and measure the final temperature
6: Use the temperature change to calculate the enthalpy change for this reaction
What is the independent variable in this experiment [the thing you change each time]
The ionic salt/solid used in the experiment
What is the dependent variable in this experiment [something you measure]
temperature change that you measure
List possible control variables that need to be kept the same in this experiment And describe briefly how you will control each of these
use the same polystyrene cup for each experiment. This eliminates differences due to heat loss in the results as a polystyrene cup and the beaker will radiate the same amount of heat/absorption ensuring reliable results.
use the same amount of moles of ionic compound/salt. Calculate the mass needed to make sure the moles are the same as the MR is fixed. because when calculating the temperature change the moles is a fixed value
use the same temperature probe for more precision. It reduces the uncertainty between readings amd a digital temp probe is even better.
When the magnetic Stirrer is placed in the polystyrene cup ensure the stirrer is stirring at a constant pace/at the same rate for all of the practicals. ensure it is kept on the same setting.
If controlled variables were not controlled how could this affect the outcome of the experiment
Purchasing all the cups from the same seller eliminates any differences. A larger cup could lead to more heat being lost therefore a less exothermic reaction
A difference in moles will not be the only dependent variable osmosis will change the value of delta H
if a different thermometer is used that will be slightly different uncertainties and increments, this decreases the reliability and increases the differences in temperature readings between each experiment. Having the same temperature probe ensures precision of temperature change measurements being the same to allow for accurate comparisons
If this stirrer is at a slower pace heat may not be distributed evenly affecting the reading on the temperature probe.
Why is a polystyrene cup used
Because it is the best insulator, it will minimize heat loss giving the closest value to the maximum temperature change. Glass and metal absorb more heat leading to less accurate results
Suggest one way that could improve the accuracy of the experiment
You could plot a cooling curve to account for heat loss
